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Most cost effective heat transfer from solar heated hot water

Posts: 157
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We are successfully heating water in 4 x 60m coils of 1/2 inch black irrigation pipe.  That this stage we have the hose at one end, and the "outhouse" shower at the other, and when the sun is on the coils it gives us hot showers.

Now we want to store that heat, and use it later.  We already have some second-hand hydronic radiators ready to install.  As we are in a farming area, we can get the 1000L  (250 Gallon) square ibc containers from $40. We thought we could use one of these to store the heat, and to run this heat through our radiators.  

We would also like to use the heat a separate water source for showers.  

We are looking at three options:

- Immerse in the tanks a large coil of the 1/2 inch black irrigation pipe.  This would be $1 per metre

- Immerse in the tanks a coil of copper pipe.  This is not cheap here but a 10mm (3/8 inch) wort coil (for beer) is available online at $100 for 7.5m or 25ft.  

- Make a separate ag-pipe coil 60m (4ft x ft) coil, connected to a retro-fix solar hot water cylinder like this one - https://www.ebay.com/itm/solar-panel-hot-water-immersion-coil-heat-exchanger-retrofit-coil-/271662256076
for the retrofit immersion copper coil, the ag pipe coil, the pump and electronic controller pulls up at around $400 if we can get a free old hot water cylinder.

I don't know how to do the maths on the heat exchange for the ag pipe and the copper coil.  Presuming we can get the water in the tank to 60C, and the water is coming into the tank about about 4C, how much submersed pipe would we need to get the water from 4C to 40C?  (Either the irrigation pipe or the copper pipe?)

Posts: 600
Location: Michigan
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Flow rate and collector output fluid temp at your flow rate are the starting point. If the flow rate is not variable, then there are calculators for different materials in coil type immersed heat exchangers. Your specific material may not be represented, a close fit will work.

I must say that after 14 years of installing manufactured systems (and some diy units), i do not do the maths for heat exchanger performance myself and rely on known performance figurest read  from manufacturers and calculators.

This is good reminder to me to learn new tricks!

Chicken scratch... i cant read any of it! But i do know that in most cases it is not likely you will use too much plastic tube. Our heat exchanger for domestic hot water is immersed in 50 gallons of purified water as storage. 250' of 1/2" pex tubing in two 125' parallel coils will not heat output water to tank temp (120-150 deg.f) on a single pass at 1.5-2 gallons per minute, but it holds enough for a shower and takes 10-15 minutes to reheat, dishes and cleaning, no problem.

Your collector loop is different, much lower flow rates 1/2-1 gallon per minute per 4'x10' collector or batch of evacuated tubes. For a small collector(s) 30'-60' of 1/2"-3/4" copper tubing has worked well over the years for us on low budget installs.

A rule of thumb i found to be usable is, plastic tubing 3 to 1 for copper in length for equvalent heat exchange.

Two good calculators and maths!





frank li
Posts: 600
Location: Michigan
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If you are in a non freezing climate, you can circulate water from the tank to the collector(s) and back to the tank. Cool thing about open (atmosphere) tanks, aside from being able to modify exchangers readily, is being able to adjust tank volume.
My favorite for this are stainless steel wine vessels with adjustable covers for tank volume.

Totally forgot this aspect. Your collector is a twofer-one, where it can be kept from freezing, unless it is seasonal use.

Your collector capacity in btu or watts is handy to know. But it sounds like you have an idea of performance.

Our shower served 5 people with one  4' x 20' pool heating collector and no additional heat exchanger. One pump (shurflo diaphram) circulated the collector circuit to drainback (freeze protection and heat scavenging) and the other pump (small centrifugal) discharged to the shower shed and outdoor sink. All plastic and from freebies and other discards aside from a customized nursery sprayer, cheap faucet and a couple fittings.

The tank is the same tank we now use indoors for a thermal storage battery and wears its original wraps of reflective "double bubble ", the thick stuff.

We manually filled the tank with a valve and used a float and flag and an indoor/outdoor thermometer for instrumentation. A salvaged, 25 year old differential thermostat from a solar air heating overhaul controlled the collector pump.

For household use we use ivan labs dc circulators for collector and boiler inputs to storage. The collector circ is pv direct and the boiler circ is battery connected through battery and pv direct capable differential thermostats from Guy Marsden. Great devices.


The boiler circuit drives purified water to the woodstove heat exchanger (<60' coil of 3/8 soft copper, drinking water safe) and back like a fountain into the tank 20' away.

Our collector loop (collector mounted, unfinished plumbing), feeds a 4'x12' glazed reynolds aluminum collector. It will not drainback, much like yours, as its risers are serpentine. This requires antifreeze or fluid evacuation in our climate. A pressurized loop that operates with extreme potential stagnation temps precludes the use of plastic plumbing, though some people do it and certain arrangements are designed to.

If our collector would drain back, we could just omit the collector heat exchanger

Our domestic hot water is pressurized by the well pump and delivered to 250' of 1/2" pex., and somewhat under-performs the way i did it, but it work for the money we have into it. It has served for about four or five years.

It needs longer coils by double or half the flow rate through it with two more coils. This will not fit well. I will calculate for a switch to copper.

Observation of feed and return temps and flow rate (or use, like shower, dog heater, sink, etc.), and time,  set up as a test bed or the maths will help determine. There is a point where space limits the practicallity of adding more plastic tube. Plastic is not a great heat exchanger generally.

Your 250 gallon tank has room for plastic, and it sounds like you have enough information to slog through the math. Keep the temps within the limits of your tank and plumbing and the water level at a reasonable level so that it reliably comes up to a usable storage temp.

This brings me to think about your "radiators" they have a thermal and likely an electrical load. They also have a temp range for usability. These want to be known before investment of materials and time.
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